# Mosfet LC oscillator

I'm looking at the following circuit:

simulate this circuit – Schematic created using CircuitLab

It consists of an oscillation whose frequency depends on the tank circuit (C1, L1 and L2). When the voltage of the tank circuit reaches 0v the mosfet that was conducting stops conducting and the one that wasn't starts conducting. This process is repeated continuously at the resonance frequency. What I don't understand is how does the voltage at node A or at node B get to 0v?

For comparison, take this circuit:

simulate this circuit

In this case there is AC through the circuit however the voltage across the capacitor is only 0 at time=0s. Afterwards, it starts increasing, but never decreasing, until it reaches the supply voltage where it would stop oscillating.

In the first circuit the continuous change of mosfets conductance results in a steady oscillation instead of it eventually dying as is the case for the second circuit. Still, I'm unable to see how nodes A/B (from the first circuit) get to 0v. I would expect that the mosfet that firsts starts conducting would charge up the capacitor to the supply voltage which would result in killing the oscillation, that however isn't the case.

Thanks

• Frequency depends on L3 as well, probably more than L1 and L2. – Andy aka Jan 16 at 17:53

Ringing...In your second circuit, once C is charged to V, there's still current flowing in L. In an ideal world, that will continue to pump C up to 2V, at which point the current reverses and it goes back down to zero. It would repeat indefinitely in an ideal world, but in the real world the AC will be damped out by resistive losses--so your circuit starts again in the other direction every cycle.

• "continue to pump C up to 2V" why to 2V? Doesn't that just depend on the dc voltage and the values of the L and C? – Huisman Jan 16 at 18:37
• Sorry for the ambiguity, I was using "V" to indicate the voltage provided by the voltage source. – Cristobol Polychronopolis Jan 17 at 13:21